A conventional automatic transmission for a vehicle includes rotating shafts that penetrate a housing. These shafts typically have bearings that rotatably support the shafts within the housing and seals are incorporated on the outside of the bearings in order to retain the transmission fluid within the housing and to keep dirt and contamination out of the housing. During operation, a conventional automatic transmission generates heat which is dissipated by the transmission fluid. A typical transmission can exceed 200° F. during normal operation. As the internal housing of the transmission is heated, the air within the housing absorbs some of the heat. As this air is heated, it undergoes a volumetric expansion and/or increase in pressure. In a sealed transmission, an increase in internal air pressure within a fluid chamber may be relieved through the shaft seals. As air escapes past the seals, it pushes some transmission fluid out through the bearings and seals to the outside of the transmission housing. This leaking of transmission fluid is undesirable, in part, because it would require additional transmission fluid to be added periodically.
Additionally, a transmission housing that has relieved air by pushing air and transmission fluid out past the bearings, will draw or suck air back in through the bearings as the air within the transmission housing cools and contracts. As air is drawn in past the seals, dirt and other contaminants can be pulled past the seals and into the bearings and fluid chamber. This contamination can shorten the life of the bearings and internal moving parts.
Therefore, to prevent an unwanted air pressure increase, air is typically vented to the outside of the transmission housing to relieve the volumetric expansion of air. Typically a vent or breather assembly is incorporated into a transmission housing in order to relieve the pressure due to this expansion of the heated air. This vent includes an elongated body inserted through an aperture in the transmission housing having a cap and a filter assembly inserted therein. The vent assemblies involve multiple machined components and typically regulate the internal pressure of the transmission equal to that of the ambient pressure outside of the transmission.
One drawback associated with this design is the inability of these vent assemblies to limit the amount of ambient air allowed into the transmission fluid chamber. As the transmission cools and air is drawn back into the transmission housing, the moisture, or humidity, within the ambient air is also drawn into the housing. As the transmission cools further, this moisture condenses within the transmission housing and mixes or stratifies within the transmission fluid. The operation of the automatic transmission can be adversely affected if an undesirable amount of water is pulled into the transmission through the vent assembly.
Another drawback associated with typical vent assemblies involves fill tube overflow. Transmission fluid is typically added to an automatic transmission through a fill tube. As fluid is added, some vent assemblies may not allow for adequate pressure relief of air to account for the volume of fluid added. When this occurs, fluid flow down the fill tube will slow and/or stop due to an air pressure increase within the transmission that exceeds the pressure exerted by the level of fluid in the fill tube. As more fluid is poured into the tube, the rate of fluid being poured can exceed the rate of fluid flow through the fill tube, causing the fluid to undesirably overflow the fill tube and spill onto the components located adjacent the fill tube.
What is needed is a transmission vent assembly that will allow an adequate amount of air pressure to be relieved and that also incorporates a relief assembly that will actuate when a pre-selected differential pressure across the transmission housing is reached.